Development of Sic Gas 

Sensor Systems 

G.W. Hunter, P.G. Neudeck, R.S. Okojie, and G.M. Beheim 

Glenn Research Center, Cleveland, Ohio 

V. Thomas and L. Chen 

Oho Aerospace Institute, Brook Park, Ohio 

D. Lukco 

QSS Group, Inc., Cleveland, Ohio 

C.C. Liu and B. Ward 

Case Western Reserve University, Cleveland, Ohio 

D. Makel 

Makel Engineering, lnc., Chico, California 

October 2002 
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Development of Sic Gas 

Sensor Systems 

G.W. Hunter, P.G. Neudeck, R.S. Okojie, and G.M. Beheim 

Glenn Research Center, Cleveland, Ohio 

V. Thomas and L. Chen 

Ohio Aerospace Institute, Book Park, Ohio 

D. Lukco 

QSS Group, Inc., Cleveland, Oho 

C.C. Liu and B. Ward 

Case Western Reserve University, Cleveland, Oluo 

D. Makel 

Makel Engineering, Inc., Chco, California 

Prepared for the 

201st Meeting 

sponsored by the Electrochemical Society 

Phladelphia, Pennsylvania, May 12-17,2002 

National Aeronautics and 

Space Administration 

Glenn Research Center 

October 2002 
Acknowledgments 

The authors gratefully acknowledge the contributions of those who made this paper and its contents possible: 

Dr. J.A. Powell, Dr. L. Matus, Dr. M. Zeller, P. Raitino, G. Fralick, Dr. M. Lienhard, Dr. J. Lei, E. Benavage, and 

L. Keys, NASA GRC; D. Androjna, B. Osborn, and J. Heisler, Akima Corporation/NASA GRC; A. Trunek and 

D. Spry, Ohio Aerospace Institute; D. Makel, Makel Engineering, Inc.; and P. Dutta, M. Frank, B. Pateon, 

J. Trimbol, and M. Fulkerson, Ohio State University. 

Available from 

NASA Center for Aerospace Information 

7121 Standard Drive 

Hanover, MD 2 1076 

National Techrucal Ildormation Service 

5285 Port Royal Road 

Springfield, VA 221 00 

Available electronically at htt~: / /eltrs.nrc.nasa .aov 
DEVELOPMENT OF' Sic GAS SENSOR SYSTEMS 

G.W. Hunter, P.G. Neudeck, R.S. Okojie, and G.M. Beheim 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 441 35 

V. Thomas and L. Chen 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

D. Lukco 

QSS Group, Inc. 

Cleveland, Ohio 44135 

C.C. Liu and B. Ward 

Case Western Reserve University 

Cleveland, Ohio 441 06 

D. Makel 

Makel Engineering 

Chico, California 95973 

ABSTRACT 

Silicon carbide (Sic) based gas sensors have significant potential to address the 

gas sensing needs of aerospace applications such as emission monitoring, fuel 

leak detection, and fire detection. However, in order to reach that potential, a 

range of technical challenges must be overcome. These challenges go beyond the 

development of the basic sensor itself and include the need for viable enabling 

technologies to make a complete gas sensor system: electrical contacts, 

packaging, and transfer of information from the sensor to the outside world. This 

paper reviews the status at NASA Glenn Research Center of Sic Schottky diode 

gas sensor development as well as that of enabling technologies supporting Sic 

gas sensor system implementation. A vision of a complete high temperature 

microfabricated Sic gas sensor system is proposed. In the long-term, it is believed 

that improveinents in the Sic semiconductor material itself could have a dramatic 

effect on the performance of Sic gas sensor systems. 

INTRODUCTION 

Silicon carbide (Sic) has high potential as the electronic semiconductor material for a 

new family of high temperature sensors and electronics. Silicon carbide can operate as a 

semiconductor and in conditions under which conventional semiconductors cannot 
adequately perform such as at temperatures above 600°C. Silicon carbide also has 

excellent thermomechanical properties and high thermal conductivity [I]. One area where 

Sic semiconductor technology can be applied is in chemical sensing. Silicon carbide gas 

sensors have been in development for a number of years using a range of designs 

including capacitors [2], transistors [3], and Schottky diodes [4-61. These sensors have 

been shown to be highly sensitive to several gases including hydrogen and hydrocarbons 

making them useful for a range of applications. 

Three aerospace applications are of particular interest [7]. The first is the monitoring 

of emissions fiom high temperature environments such as combustion systems or 

chemical processing reactors. In both aeronautic and commercial emissions monitoring 

applications, the detection of hydrocarbons in a mixed chemical environment can be used 

to reduce emissions and potentially monitor the efficiency and health of the engine. 

Further, the monitoring of emissions from chemical processing reactors or on-board 

experiments associated with Space Shuttle or International Space Station operation can 

be used to provide early warning if a system is malhnctioning or to enhance the 

experiment. 

A second application is the monitoring of fuel leaks in launch vehicles. Detection of 

low concentrations of hydrogen and hydrogen-based fuels is important to avoid ex.ploslve 

conditions that could harm personnel and damage the vehicle. Reliable vehicle operation 

also d.epends on the timely and accurate measurement of these lea,ks. Further, 

measurement of fuel gases such as hydrazine at low concentrations is desired due to their 

high toxicity and potential to contaminate the environment in the space station. 

A third application is fire detection on-board commercial aircraft and space vehicles. 

Rapid detection and location of a fire is extremely important to avoid catastrophic 

situations. While existing smoke detectors are able to detect fires, they have a high 

incidence of false alarms and thus a second, independent method of fire detection to 

complement conventional smoke detection techniques is required. The measurement of 

chemical species indicative of a fire is envisioned to help reduce false alanns and 

improve system safety. While carbon monoxide (CO) and carbon dioxide (COz) are 

important, information derived from other gases such as hydrocarbons is also of interest. 

These applications require operation in a variety of conditions: from cryogenic 

temperature to above 600°C, from inert environments to highly corrosive engine 

conditions, and from the detection of one gas over a wide concentration range in inert 

environments to the detection of several gases over more narrow concentration ranges in 

the presence of interfering gases. The sensor design and sensing approach depends 

strongly on the specific application. Further, supporting technologies like interconnects 

and packaging, without which the sensor would be inoperable, need to be tailored for the 

application. A complete, operational sensor system includes not only the sensor itself but 

also includes supporting electronics, micromachined components, packaging and 

interconnects. For many applications, the sensor system components would be required to 

reliably function over long operational lifetimes. While this may be less problematic for 
predominately room temperature applications like leak detection, the challenges are 

considerable in high temperature applications such as engine monitoring. 

This paper will give an overview of the on-going technology development at NASA 

Glenn Research Center (GRC) to produce Sic-based gas sensor systems for a range of 

aeronautic and industrial applications. This includes not only the development of the 

sensor but also the supporting technologies that make the sensor system operational in a 

given environment. First, an overview of the sensor development will be given. Then a 

review of the state of development of enabling technologies necessary for Sic sensor 

systems is presented including: 1) Supporting high temperature Sic electronics, 2) High 

temperature contacts, 3) Micromachining technologies, and 4) High temperature 

packaging. The use of the Sic-based sensor in engine environments as part of a High 

Temperature Electronic Nose will be discussed. A vision of how these individual 

components can be integrated into a Sic gas sensor system will be presented. While work 

proceeds towards improving long-term durability at high temperatures and realizing the 

high potential of these systems, Sic gas sensor systems are still operational for a range of 

lower temperature applications. One significant future direction of this work is 

improvements in the starting Sic semiconductor material. 

SENSOR DEVELOPMENT 

The development of SIC gas sensors at NASA GRC has centered on the development 

of a stable, gas-sensitive Sic-based Schottky diode. A Schottky diode is composed ,)f a 

metal in contact with a semiconductor (MS) or a metal in contact with a very fnin 

insulator or oxide on a semiconductor MIS or. MOS). For gas sensing applications, the 

metal is often a catalytic film. The advantage of a Schottky diode sensing structure In gas 

sensing applications is its high sensitivity. This is especially usefin1 in applications such as 

emission measurements where the concentrations to be measured are low (on the order of' 

ppm). The detection mechanism for hydrogen (Hz) involves the dissociation of H2 on the 

surface of a catalytic metal leading to the formation of a dipole layer at the interface of 

the metal and the insulator (or metal-semiconductor interface depending on the structure). 

This dipole layer affects the effective Schottky barrier height of the diode resulting in an 

exponential change in the forward current and a quadratic change in the capacitance 18-91 

while the diode is under bias. The detection of hydrocarbons is possible if the sensor is 

operated at a high enough temperature to dissociate the hydrocarbon and produce atomic 

hydrogen. The resulting atomic hydrogen affects the sensor output in the same way as 

molecular hydrogen [4, 10-1 11. 

The Schottky diode structure under development at NASA GRC began with 

palladium (Pd) thin film on Sic MS structures (PdJSiC). Direct contact between tbe 

catalytic metal and the semiconductor allows changes in the catalytic metal to have 

maximum effect on the electrical properties of the Schottky diode. The details of this 

work are reviewed elsewhere [4]. The sensor detects hydrogen and hydrocarbons in inert 

or oxygen containing environments. Figure 1 illustrates one main advantage of Sic over 

silicon (Si) semiconductors in Schottky diode hydrocarbon sensing applications [4]. 
The zero bias capacitive response of a Pd/SiC Schottky diode to one hydrocarbon, 

propylene, over a range of temperatures is shown. The sensor operational temperature is 

increased from 100°C to 400°C in steps of 100°C and the response of the sensor is 

observed. At a given temperature, the sensor is exposed to air for 20 minutes, N2 for 

20 minutes, 360 ppm of propylene in N2 for 20 minutes, Nz for 10 minutes, and then 

10 minutes of air. The magnitude of sensor response to 360 ppm propylene depends 

strongly on the operating temperature. A sensor operating temperature of 100°C is too 

low for propylene to dissociate on the Pd surface, so the device does not respond at all. 

The three other curves for 200°C, 300°C, and 400°C show that elevating the temperature 

increases the sensor's response to propylene. The presence of propylene can be detected 

at any of these higher temperatures with 200°C being the minimum operating temperature 

determined in this study. Since the operating temperature of Si Schottky diodes is 

typically confined below 200°C, these results demonstrate the significant advantages sf 

using Sic diodes rather than Si diodes in some gas sensing applications. 

Figure I. The temperature 

dependence of the zero 

bias capacitance to various 

gas mixtures for a PdISiC 

Schottky diode. 

TEMPERATURE 

a 100c 

A 200 C 

However, the sensor response is adversely degraded by extended high temperature 

heating. Prolonged heating at 425°C has been shown to change the sensor properties and 

to decrease sensor sensitivity 141. The reason for this change in diode properties is 

thought to be due to reactions between the Pd and Sic at the interface upon heating 

causing disruption of the metal structure due to oxygen (Oz) and Si [12-131. Efforts are 

underway to stabilize the sensor structure for long-term, high temperature operation. 

Two structures have been demonstrated to improve the stability of the Pd-based 

Schottky diode structure over that of the simple Pd/SiC Schottky sensor [5]. Tlie first 

structure involves the incorporation of chemically reactive oxides into the Sic-based 

Schottky diode structure. A wide variety of materials, e.g. metal oxides such as SnO?, are 

sensitive to hydrocarbons (CXH,) and nitrogen oxides (NO,) at high temperatures. 
These materials could be incorporated as a sensitive component into MOS structures. 

Unlike silicon, Sic-based devices can be operated at high enough temperatures for these 

materials to be reactive to gases such as hydrocarbons (CxH,) and nitrogen oxides (NO,). 

This results in a different type of gas sensitive structure: a metal reactive-oxide 

semiconductor structure (MROS). The advantages of this type of Sic-based structure 

include 1) increased sensor sensitivity since the diode responds to gas reactions due both 

the catalytic metal and the reactive oxide, 2) improved sensor stability since the gas 

reactive oxide can act as a diffusion barrier layer between the metal and Sic potentially 

stabilizing the sensor's interfacial properties, and 3) the ability to vary sensor selectivity 

by varying the composition of the reactive oxide element enabling the fabrication 

Sic-based arrays for high temperature gas detection. (See Summary and Future Directions 

discussion below). 

The second structure is PdCr directly deposited on Sic (PdCrISiC). The advantages 

of PdCr as a high temperature alloy have been explored extensively in strain gage applications [14]. It is a stable high temperature material able to provide static strain 

measurements at temperature up to 1 100°C. However, its use in a gas-sensing Sic-based 

structure depends on not only its inherent stability but also on such factors as the alloy's 

reactivity to Sic and the catalytic interactions of PdCr alloy with the gases to be 

measured. Results have been presented previously which suggest that PdCr can form a 

stable system that has high sensitivity to hydrogen (-4 orders of magnitude change for 

120 ppm H2 in N2 at 100°C) even after 240 hours of heating at 425OC in air [S]. The 

results of surface and interface analyses indicated that after testing (or heating) part of 

PdCr thin film was still clean (free of Si) and the sensor surface was also relatively clean 

(lower silicon oxides contents) compared with those of Pd/SiC sensors examined [I 51. it 

was suggested that a carbide component at the interface region may be critical in 

maintaining sensor performance stability and sensitivity. 

However, despite these promising results, the ability to systematically produce films 

with combined stability and sensitivity has been problematic. One potential reason for 

this is the lack of consistency of the Sic surface (See Summary and Future Directions 

discussion below and reference 1). Recent results have shown that PdCr films of various 

PdfCr ratios show varying levels of reactivity with the Sic surface as well as sensor drift 

after extended durations at high temperature. A representative Auger depth profile is 

shown in Figure 2 of a PdCr film heated at 450°C for 100 hours and periodically exposed 

(an average of at least 20 minutes every 24 hours) to 0.5% hydrogen in nitrogen. Two 

major trends have been found to correspond to a decline in sensor response: Pd migration 

into the Sic interface to form of palladium silicides (PdSi,) and the subsequent migration 

of elemental silicon to the surface from the Sic. Palladium silicides are present 

throughout the film and the Si on and near the surface oxidizes forming silicon dioxide. 

Accompanying this silicide and oxide formation are the migration of carbon (C) into the 

PdCr from the Sic and of Cr into the Sic. In summary, significant migration of the 

components of the PdCrISiC interface occur with high temperature heating. Similar 

silicide and oxide formations have been noted in Pd/SiC diodes heated at 425OG [B2]. 

Thus, the use of the PdCr alloy alone on Sic is not sufficient to produce stable and 
sensitive sensors. Control of the migration of the constiuents of the PdCrISiC including 

Pd, Si, and Cr as well as the subsequent formation of silicides and oxides is necessary to 

achieve improved stability. 

Figure 2. AES depth 

profile of a 

representative PdCrISiC 

diode after periodic 

0.5% hydrogen in 

nitrogen gas exposure 

for 100 hrs at 450 "C. 

0 200 400 600 800 qW 

DEPTH (A) 

In order to stabilize these films at higher temperatures, various approaches have been 

investigated. A combination of thin film layering, thermal pre-treatment, and surface 

coating of the Sic substrate has been used. One approach involves the deposition of 

multilayer thin films deposited on n-type, C-face 6H-SiC substrates with a 3 pm epilayer 

obtained from Cree Research [16]. The sample was cleaned and pretreated with a thiol. 

compound prior to metal layer depositions. The thin film combination of Pd 

(1 75 A)/Cr(35 A)/PdCr(13%)[4 10 AllCr(70 A) was deposited via sputtering using an 

argon plasma at pressures in the mTon range. The wafer was diced and a sample was 

treated at 450°C by cyclically varying the ambient between, air, nitrogen, and exposure 

(while biased) to hydrogen at a concentration of 0.5% in a balance of nitrogen gas. Auger 

depth profiling was performed on both an as-deposited and treated film. Further details of 

the sample preparation and analysis is planned for a hture publication. 

The results of the AES analysis of an as-deposited film are shown in Figure 3 for the 

as-deposited as well as that for a treated film. In both curves, the inflection point in the Si 

and C profile curves occurs at about 800 A and the width of the interfacial region is 

nearly invariant. This indicates a thermally stable interface even though limited 

interfacial diffbsion of Pd, Cr, and 0 occurred. In the as-deposited film, the various Pd, 

Cr, and Pd Cr layers are defined by rapid changes in the elemental compositiors with 

slight broadening of the Cr distribution near the interface. After treatment, Pd and Cs 

redistribute throughout the film. Palladium and Cr have a nearly uniform distribution in 

the film while Pd has only a slightly enhanced concentration at the interface and into the 

Sic. The AES data for the Si region definitively shows that the migrated Pd species is 
metallic and no evidence of any silicide was found even on the surface where significant 

Si02 had previously formed. Oxygen distributes through the film and to some degree - 

through the interface. An accumulation of CrICr oxide species near the surface is not 

observed. It should be noted that this approach has apparently eliminated oxygen 

'leaching' of Cr from the PdCr alloy film. 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., 

Figure 3. AES depth profile 

of the Pd/Cr/PdCr(l3%)/Cr 

annealed sensing film: 

a) as deposited. 0 200 400 600 808 IW 

b) after annealing at 450°C 100 EP-rH(4 

for approximately 100 hrs -9 with periodic hydrogen $- 

80 gas exposure. z 

0 F 70 2 60 

E 50 

0 

Z 

om 

0 

0 30 

g 20 

2 10 

0 

0 200 400 600 800 1m 

DEPTH (4 

Thus, a combination of thin film layering and surface treatment of the Sic substrate 

has been effectively employed to improve the stability of PdCr sensing films on Sic 

substrates especially with respect to Si migration. Even after over 100 continuous hours 

at 450°C, the metal-Sic interface remains predominantly intact and Si migration to the 

surface is not observed. Further, Cr accumulation at the surface is also eliminated. 
This behavior is in stark contrast to that of the PdCrISiC sample seen in Figure 2. The 

data suggests that treatment of the surface with the thiol compound decreases the 

chemical potential of the surface below that for the untreated Sic surface and reduces the 

Pd and Cr silicide formation. However, preliminary sensor response testing suggests that, 

while less Si diffusion has taken place, this sample may have significantly reduced 

sensitivity compared to that discussed in, for example, reference 5. 

Further efforts are underway to produce a Sic Schottky diode sensor that is both 

stable and sensitive at high temperatures. However, it should be noted that even though 

the sensor drifts at temperatures such as 450°C, this does not preclude its use at lower 

temperatures for some applications. While some applications require high temperature 

exposure (e.g., in-situ engine monitoring) or some gases need higher temperatmes to 

decompose for detection (e.g., methane), other applications are less demanding and thus 

may be open for use of this sensor in its present stage of development. Further, if the 

application only requires short operation times, then use of the present sensor is feasible. 

(See High Temperature Electronic Nose discussion below for a limited duration 

demonstration of the Sic-based gas sensor.) 

ENABLING TECHNOLOGIES 

The long-term goal is to establish a complete Sic-based gas sensor systems able to 

meet the needs of a range of applications. The most challenging applications are those 

that require in-situ measurements in harsh environments e.g. aeronautic or commercial 

engine applications. However, even applications which do not normally require harsh 

environment operation may want the system to remain operational if the environment 

becomes harsh e.g., fire detectors which operate at room temperature may need to be 

continuously operational after a fire breaks out and the temperature rises considerably. 

The following is a discussion of the state of maturity of some of the enabling 

technologies meant for harsh environment applications that would support the Sic-based 

gas sensor and contribute to a complete sensor system. For more moderate applications, 

other technologies may be used and even preferred for cost reasons. 

High Temperature Electronics 

Silicon carbide presently appears to be the strongest candidate semiconductor for 

implementing 500 to 600°C integrated electronics in the nearer term [I]. Sic electronics 

are envisioned to enable signal conditioning and wireless communication and thus could 

be used to complement Sic gas sensor technology in this temperature range. Discrete SIC 

devices such as pn junction diodes, junction field effect transistors (JFET7s), and metaloxide-semiconductor field effect transistors (MOSFET7s) have previously demonstrated 

excellent electrical fbnctionality at 600°C for relatively short time periods [I]. However, 

for such electronics to be useful in high temperature applications, much longer 600°C 

harsh-environment lifetimes must eventually be realized. The operational lifetime of 

Sic-based transistors at 600°C is not limited by the semiconductor itself, but is instead 

largely governed by the reliability and stability of various interfaces with the Sic crystal 

surface. The physical degradation of the metal-semiconductor ohmic contact interface 
limits the 600°C operating lifetime of all devices, while high temperature MOSFET 

operating lifetime is also limited by the electrical integrity of the oxide-semiconductor 

interface. Thus, junction-based transistors without gate insulators appear more feasible in 

the nearer term. Of the candidate junction-based transistor technologies that might be 

used to implement Sic integrated circuits, the pn junction gate JFET seems closest to 

demonstrating long-term operation at 600°C. 

An example of the maturity of the JFET technology is the demonstration of 600°C 

digital logic using Sic JFET's [17]. A resistive load direct-coupled FET logic (DCFL) 

approach was adopted to demonstrate simple 600°C digital logic using Sic JFET's. The 

non-planar epitaxial gate JFET design was adopted in favor of a planar ion-implanted 

structure, largely to alleviate the challenging process of sufficiently activating high-dose 

p+ ion implants in Sic. A two-level interconnect approach using oxidation-resistant 

silicon nitride as the dielectric passivant along with oxidation resistant gold for the metal 

interconnect also adopted. Figure 4 shows operation of NAND gate at 600°C. The 

devices show good operation at these high temperatures as shown in the figure. These 

circuits are the fundamental building blocks of more complex systems and therefore the 

capability of building high temperature circuits to complement Sic gas sensor technology 

potentially exists. However, degradation of contacts to the device in Figure 4 limited 

operational circuit testing at 600°C to less than one hour. 

Figure 4. NAND gate test 

waveforms at 600°C with 

VDD = 2.5 V, Vss = 0 V, 

Vsubstrate = -1 -4 V. 

0 2 4 6 8 

TIME (msec) 

High Temperature Contacts 

The operational lifetime of circuits operated at 600°C could be extended by using 

separately optimized n-type and p-type ohmic contacts specifically designed to resist hightemperature degradation. Significant progress has been made in improving the reliability of 

high temperature contacts [18]. A multilayer approach using Ti (lOOnm)/TaSiz 

(200nm)/Pt(300nrn) on n-type 6H-Sic players has resulted in a thermally stable ohmic 

contact metallization at temperatures up to 600°C. These contacts exhibited linear ohmic 
characteristics with contact resistance in the range of (0.3~10~ 8x10~ a-cmz) on n-type 

epilayer doped between 0.6 to 2x10'~ cmp3. AES was used to analyze the metal and 

semiconductor interfaces to understand the prevailing reactions. The thermal stability of the 

ohmic contacts between 500°C and 600°C in air is believed to be due to the formation of 

silicides and carbides of titanium after being annealed during a pretreatment at 600°C in 

Hz (5%)/Nz forming gas for 30 minutes. The oxidation of Si species that migrated after 

TaSiz decomposition is the most probable diffusion barrier mechanism. 

This metallization approach has been demonstrated to result in thermally stable ohmic 

contact at 600°C in air for 1000 hours as seen in Figure 5 where the average specific 

contact resistivity is shown for both 6H and 4H samples. The Ti/TaSiz/Pt contacts have 

low resistivity and high stability after an initial break-in period. Therefore, a metallization 

scheme exists to improve the durability of Sic electronics for up to 1000 hours at 600°C. 

Future work will demonstrate this contact technology in integrated Sic electronics 

circuits. Given the high stability of this metallization system, an obvious approach to 

resolution of the gas sensor stability/sensitivity issues discussed above might be the use 

this metallization scheme as a gas sensor. However, preliminary results have suggested 

that this system is generally not sensitive to hydrogen and hydrocarbons. 

t. n-type 6H-SiC 

Figure 5. Average + n-type 4H-SiC 

specific contact 

resistivity of 

Ti /TaSiz /Pt contacts as 

a function of heat 

treatment at 600°C for 

both 6H and 4H 

samples. 

0 100 200 300 400 500 600 750 800 900 1000 

TIME (hrs) 

Micromachininn Technology 

The use of micromachining in chemical sensors has several advantages. By 

combining micromachining technology with microfabrication techniques, threedimensional chemical sensing structures can be formed. In Si, the sacrificial layer method 

is often used [19]. The sacrificial layer method employs a deposited underlayer that can 

be selectively removed chemically. This results in a suspended diaphra,m structure with 

a small thermal mass on which the sensor, temperature detector and heater can be 

fabricated. This yields a sensor structure with higher thermal stability, minimal heat loss, 

and less energy consumption. Figure 6 shows the use of a diaphragm in the struclure in 
an oxygen sensor design. The fabrication of this structure depends on chemical 

anisotropic etching of Si. In order to achieve such a design in Sic, controlled etching 

processes for Sic are necessary. 

ZIRCONIUM 

PLATINUM OXIDE 

SILICON 

NITRIDE 

DIAPHRAGM K R:::::~ A 

SILICON 

DIOXIDE 

Figure 6: The structure of a Figure 7: Scanning Electron Microscopy 

microfabricated amperometric oxygen (SEM) micrograph of cavity etched in 

sensor based in Si showing a diaphragm 6H-Sic by deep reactive ion etching 

structure. method. 

However, the fabrication of a diaphragm structure using chemical etching in Sic is 

problematic [1,20]. Due to the chemical inertness of Sic, standard etching processes are 

ineffective. A deep RIE process using an inductively coupled plasma (ICP) etch system 

has been developed which provides anisotropic etch profiles and smooth etched surfaces, 

a high rate (2200 Almin), and a high selectivity (55:l) to the nickel etch mask. This 

results of this process are shown in Figure 7 where a cavity is etched in Sic with an etch 

depth of 245 pm. Further work is needed to optimize the surface cleaning procedure 

which precedes the deep etch process. In addition, work is planned to develop a process 

which produces smoother sidewalls, which are important in some applications, such as 

for mesas for vertical structure high-voltage devices. Nonetheless, the fabrication of this 

cavity, intended for pressure sensing applications, demonstrates the base capability to 

perform deep Sic semiconductor etching. As application needs arise for minimizing the 

thermal mass of Sic gas sensor structures, the same type of technology could be used to 

fabricate diaphragm structures for Sic gas sensing applications. 

High Temperature Packaging 

The operation of chemical sensors in high temperature environments requires 

packaging technologies beyond those of conventional device packaging technology. For 

example, for in sitz~ engine monitoring of chemical species, sensors must operate at 

temperatures of at least 500°C and at gas turbine pressures up to 3000 pounds per square 

inch (psi). This is a chemically reactive gas environment composed of chemical species 

such as oxygen in air, and hydrocarbonshydrogen in fuel, and species such as NO, in the 
combustion products. Additionally, the sensors and associated electronics may be 

exposed to vibration. Thus, the packaging materials and basic components must 

withstand temperatures of at least 500°C, chemical corrosion, and potentially high 

dynamic pressure and high acceleration. These packaging components include the 

substrate, metallization material(s), electrical interconnections within the package (such 

as wire-bond), and die-attach. These requirements are far beyond standard operation 

conditions for most advanced (commercial) packaging and thus new technology 

development is necessary. 

The most promising technology for such applications are ceramic subs-lrates 

combined with precious metal thick-film metallizations. These technologies have been 

proposed for hybrid [2 11 and chip-level packaging of high temperature operable 

microsystems [22] based on their excellent stabilities at high temperature and in 

chemically reactive environments. Aluminum nitride was proposed to package high 

temperature Sic MEMS because this material possesses a low thermal expansion 

coefficient which is close to that of Sic [22-231. 

An innovatively designed, fabricated, and assembled chip-level electronic package 

for high-temperature harsh environment microelectronic systems using ceramic 

(aluminum nitride and aluminum oxide) substrates and gold (Au) thick-film metallization 

(see Figure 8) has been produced. The electrical interconnection system of this advanced 

packaging system, including the thick-film metallization and wirebond, has been 

successfilly tested at 500°C in an oxidizing environment for over 5000 hrs. A compatible 

low resistance die-attach scheme using Au thick-film material as a conductive bonding 

material has also been developed to package Sic rnicrosystems. 

This complete electrical interconnection system was tested using an NASA CRC: 

fabricated Sic semiconductor test chip (Schottky diode with a gold contact) in an 

oxidizing environment in a temperature range from room temperature to 500°C for more 

than 1000 hrs. The thick-film-based interconnection system demonstrated the required 

low (2.5 times of the room-temperature resistance of the Au conductor) and stable 

(3 percent decrease in the first 1500 hrs of continuous test) electrical resistance at 500°6 

in an oxidizing environment. Also, the electrical isolation impedance between the printed 

wires that were not electrically joined by a wirebond remained satisfactorily high 

(>0.4 GS2) at 500°C in air. The attached Sic diode demonstrated low (< 3.8x1W2 

S2- cm2) and relatively consistent forward resistance from room temperature to 500°C, as 

shown in Figure 8. This implies the operability of the die-attach between room 

temperature and 500°C. Preliminary results from finite element analysis of a Sic dieattach with lmm2 die on an aluminum nitride substrate show low thermal stress [24]. 

These results indicate that this prototype package and the compatible die-attach scheme 

meet the basic requirements for low-power, long-term operation in high-temperature and 

chemically reactive environments. This base technology can be then tailored for 

individual chemical sensing applications. For example, the package design shown in 

Figure 8 seals the device. This approach is appropriate for electronic devices to support 

the chemical sensors, while a chemical sensor package would expose the sensor itself. 
Figure 8. a) Prototype high-temperature electronic package composed of ceramic 

substrates and Au thick-film metallization for harsh environment systems. b) Forward 

resistance of a packaged Sic Schottky diode characterized at 500°C in oxidizing 

environment for over 1000 hrs. 

Sic GAS SENSOR APPLICATION: 

HIGH TEMPERATURE ELECTRONIC NOSE TECHNOLOGY 

An example of the integration of Sic gas sensors into a system is the inclusion of the 

sensor in the High Temperature Electronic Nose. The fbnction of the High Temperature 

Electronic Nose is to characterize the chemical signature of the emissions in a complex 

high temperature chemical environment. The characterization of such an environment is 

difficult with a single sensor due to issues of sensor selectivity and cross-sensitivity 171. 

Rather, an array of sensors optimized for selectivity to specific gases is necessary for the 

characterization of the components of a mixed environment containing, for example, 

C,H,, 02, and NO,. Development of a High Temperature Electronic Nose consisting 01 

such a microfabricated gas sensor array combined with data processing software and 

operable at high temperatures and high flow rates would be a dramatic step towards 

realizing the goal of monitoring/control of emissions produced by an engine, a power 

generation unit, or a chemical reactor. 

The elements necessary for the High Temperature Electronic Nose include those that 

have been discussed in this paper (although not all the components are presently high 

temperature compatible). A Sic Schottky diode gas sensor is included in this array; its 

output signal is complemented by other microfabricated sensors to allow differentiation 

of several gases. This microfabricated array is mounted into a package and placed in the 

high temperature environment. Interconnections are then made from the sensor anray to 

standard electronics mounted outside of the hot area where the Nose operates. 
The electronics process the signals from the sensors as well as control their temperatures 

with heaters and temperature detectors. In the future, this gas response will be integrated 

and interpreted using neural net processing, currently being developed by Ohio State 

University to allow a more accurate determination of the chemical constituents of harsh, 

high temperature environments. 

This first generation High Temperature Electronic Nose has been demonstrated on a 

modified automotive engine [25]. Figure 9 shows the response of a Sic-based (CXH,) 

sensor, tin oxide based sensor (doped for NO, sensitivity), and an 02 sensor at various 

engine operation stages. The figure shows the individual sensor responses during the 

initial start of the engine, a warm-up period, a steady state operation period, and at the 

engine turn-off. The sensors were operated at 400°C while the engine operating 

temperature was 337°C. Each sensor has a different characteristic response. The oxygen 

sensor shows a decrease in 02 concentration while the NOx and CxHy concentrations 

increase at start-up. The hydrocarbon concentrations decrease as the engine warns up to 

steady-state while the NO, concentration increases before stabilizing. The Oz, NO,, and 

CxHy concentrations all return to their start-up values after the engine is turned off. These 

results are qualitatively consistent with what would be expected for this type of engine. 

They also show the value of using sensors with very different sensing mechanisms in all 

electronic nose array: the information provided by each sensor was unique and monitored 

a different aspect of the engine's chemical behavior. 
SUMMARY AND FUTURE DIRECTIONS 

This paper has presented an overview of the status of development of the components 

of a potential Sic gas sensor system operable at high temperatures. While the system 

would have to be tailored for the given application, an envisioned Sic-based gas sensor 

system for high temperature, harsh environments could include the following 

components: A Schottky diode-based gas sensor for .highly sensitive detection with, 

depending on the application, complementary sensors for different gases and 

concentration ranges. For example, gas sensitive resistors could be included on the same 

Sic substrate for higher concentration measurements while sensors of significantly 

different operating principles could be combined with the Sic gas sensors (High 

Temperature Nose approach) to characterize mixed chemical environments. The SIC gas 

sensors themselves can be tailored for the application by using different types of alloys or 

MROS structures for improved sensitivity. Physical sensors (such as pressure sensors) 

could also be included depending on the application. Sic electronics could be integrated 

with sensors for signal conditioning and onsite data processing. Wireless communicatioil 

would eliminate the need for wires (although for a truly wireless system, on-board power 

would also be necessary). The sensor structure would be micromachined for mi~~imal 

thermal mass and power consumption and include an integrated temperature detector and 

heater for temperature control. High temperature packaging would allow system 

operation in harsh environments with sufficient protection for the high temperature 

electronics while allowing the exposure of the sensors to the operational environment. 

The components of this system are in various stages of maturity as discussed in this 

paper. The Sic-based gas sensor has shown promise but long-term, high temperature 

operation is still problematic. Lower temperature operation of these sensors is still a 

viable option for a number of applications. The contact and packaging technologies have 

been shown to be reliable at high temperatures for extended periods with micromachining 

capabilities available. The electronic device technology operational lifetime can be 

enhanced by the improved contact and packaging technologies. Further development will 

be necessary before these electronics can be integrated into a Sic-based gas sensor 

system to provide data processing and communication. In the meantime, remote location 

of components such as electronics, as demonstrated with the High Temperatwe 

Electronic Nose, is a viable option until complete integrated systems can be fabricated for 

high temperature, harsh environments. 

However, the technology advancement which may have the strongest impact on %be 

maturation of Sic gas sensor systems (sensor and enabling technologies) is improved SIC 

electronic materials. Compared to silicon wafer standards, present-day Sic wafers are 

small, expensive, and generally of inferior quality. In addition to high densities of 

crystalline defects such as micropipes and closed-core screw dislocations, comrnerciaP 

Sic wafers also exhibit significantly rougher surfaces, and larger warpage and bow than 

is typical for silicon wafers [26]. Further, problems with the quality of the oxide growth 

on Sic have prevented realization of Metal-Oxide-Semiconductor Field Effect 

Transistors (MOSFET) technology in Sic. The vast majority of semiconductor integrated 
circuit chips in use today in silicon rely on MOSFET technology. The disparity is not 

surprising considering that silicon technology has undergone several decades of 

commercial process refinement. While significant advances have nonetheless been made 

in the development of Sic sensors and electronics, the existence of improved SIC starting 

material is expected to significantly enhance the development of these technologies. 

Recent work has been aimed at improving the quality of the Sic starting material on 

which devices are fabricated by eliminating the defects and growing superior SIC 

surfaces on device-sized mesas. The formation of Sic mesa surfaces as large 0.2 by 

0.2 mrn completely free of a single atomic step was recently reported [27]. W-ork is 

presently on-going to take advantage of the properties of these step-f?ee surfaces in a 

manner that could enable advantageous new electrical devices to be realized and the 

realization of complete, harsh environment Sic gas sensor systems. 

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Development of Sic Gas Sensor Systems I 

G.W. Hunter, P.G. Neudeck, R.S. Okojie, G.M. Beheim, V. Thomas, L. Chen, 

D. Lukco, C.C. Liu, B. Ward, and D. Make1 

REPORT NUMBER 

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135 -3 19 1 

National Aeronautics and Space Administration 

Washington, DC 20546-0001 NASA TM-2002-2 1 1707 

G.W. Hunter, P.G. Neudeck, R.S. Okojie, and G.M. Beheim, NASA Glenn Research Center; V. Thomas and L. Chen, 

Ohio Aerospace Institute, Brook Park, Ohio; D. Lukco, QSS Group, Inc., Cleveland, Ohio; C.C. Liu and R. Ward. Ci~e 

Unclassified - Unlimited 

Subject Category: 35 Distribution: Nonstandard 

Available electronically at htt~://pltrs.erc.nasa.pov 

Silicon carbide (Sic) based gas sensors have significant potential to address the gas sensing needs of aerospace appllca 

tions such as emission monitoring, fuel leak detection, and fire detection. However, in order to reach that potent~al. a 

range of technical challenges must be overcome. These challenges go beyond the development of the baslc sensor itseli 

and include the need for viable enabling technologies to make a complete gas sensor system: electrical coiitacts. packaging, and transfer of information from the sensor to the outside world. This paper reviews the status at NASA Glenn 

Research Center of Sic Schottky diode gas sensor development as well as that of enabling technologies sulpportmg SIC 

gas sensor system implementation. A vision of a complete high temperature microfabricated Sic gas sensor systein IS 

proposed. In the long-term, it is believed that improvements in the Sic semiconductor material itself could h dve a 

dramatic effect on the performance of Sic gas sensor systems. 

Gas; Leak; Emissions; High temperature; Electronic; Packaging; Detection 16. PRICE CODE 

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